Counterbalancing systems for sectional overhead doors have commonly employed torsion spring arrangements. The use of torsion springs in such sectional overhead doors is, in significant part, because the linear tension characteristics of a torsion spring can be closely matched to the substantially linear effective door weight as a sectional door moves from the open, horizontal position, where the door is largely track supported, to the closed, vertical position or vice versa. In this manner, the sum of the forces acting on such a sectional garage door may be maintained relatively small except for momentum forces generated by movement of the door by the application of manual or mechanical forces. In this respect, sectional overhead doors have been provided with lift cables or similar flexible elements attached to the bottom of the door and to cable storage drums mounted in spaced relation on a drive tube, which rotate when the drive tube is actuated.
In many cases, these cable storage drums have surface grooves that guide the lift cables on and off of the cable storage drum to prevent the coils or cable wraps from rubbing against each other and chafing which would occur if positioned in side-by-side engaging relationship or if coiled on top of each other. Lift cables sized to meet operational requirements for sectional overhead door applications are commonly constructed of multiple strand steel filaments that have a pronounced resistance to bending when stored on the circumference of the cable drums and, thus, require tension to remain systematically coiled or wrapped about the cable drums in the surface grooves therein.
A problem arises if tension is removed from one or both of the lift cables of a sectional overhead door in that the lift cables tend to unwrap or separate from the cable drums; thereafter, when tension is restored, the lift cables may not relocate in the appropriate grooves or in appropriate relation to adjacent cable wraps. In some instances, a cable wrap will locate on a groove further axially inboard of the door from its original position so that as the door moves to the fully opened position, the cable drum runs out of grooves for cable wraps, such that the lift cable coils about parts of the drum that are not designed for cable storage. In this instance, if the lift cable dislodges from the cable storage drum and engage the smaller radius of the counterbalance system drive tube, the leverage affected by the springs through the cable drum and cable is reduced such that the door will be extremely difficult or impossible to move. This is because the linear force between the door and the counterbalance springs relies on the leverage against the counterbalance spring being applied by the weight of the door operating through the radius of the cable storage drum grooves rather than a reduced radius portion of the cable drum or the drive tube for the counterbalance system
In other instances, the removal of tension from the lift cables can result in cable wraps or coils being axially displaced from the proper groove on the cable drum to overlie existing cable wraps stored on the cable drum, which may cause the length of cable between the cable drums at opposite ends of a door to assume a different effective operating length. In such case, the door may be shifted angularly in the door opening, with the bottom edge of the door no longer paralleling the ground and the ends of the door sections moving out of a perpendicular orientation to the ground. When thus angularly oriented, continued movement of the door can readily result in the door binding or jamming in the track system and, thus, being rendered inoperative.
In the instance of either of these operating anomalies occasioned by loss of tension in the lift cables, it is probable that the resultant tangling of the lift cables and/or jamming of the doors will prevent the door from further automatic or manual operation, leave the door in a partially open condition, and require qualified service personnel to repair or replace damaged components and reassemble and realign the door and counterbalance system components before the door is restored to normal operating condition.
There are a number of possible operating circumstances wherein tension in the lift cables of a counterbalance system for a sectional overhead door becomes reduced to such an extent that the lift cables may become mispositioned on or relative to the cable storage drums, thereby producing the problems discussed above. One example is when a door is rapidly raised from the closed to the open position at a velocity that is faster than the cable storage drums can rotationally react, such that slack is created in the lift cables. Another example is in the utilization of a motorized unit, such as a jackshaft type operator, that turns the counterbalance system shaft to open and close a sectional overhead door. A jack-shaft may create cable slack when the operator turns the cable storage drums without the door moving. Many jackshaft operators have motor controls and sensors that will determine if the operator is turning the counterbalance tube without the door moving to minimize cable slack which will result in the cables becoming entangled. However these methods are not exact nor are they instantaneous such that the operator could rotate the drive tube and cable drums through one or more revolutions before the sensors signal the motor controls to shut the motor off. During this time the cable is slack and if this occurs when the door is in the fully open position, the cables can become tangled preventing further movement of the door.
One approach to preventing cable mispositioning has involved utilization of grooves in the circumference of the cable storage drums, which are otherwise present for positioning and spacing cable as it is taken up during the raising of a garage door. In some instances, exaggerated or deep grooves have been employed in the cable storage drums in an effort to maintain the lift cables appropriately positioned during a loss of tension on the lift cables. While the use of grooves so configured may be helpful in preventing lift cable mispositioning in minor losses of tension, this approach does not solve the commonly encountered problem of appreciable slack being created in the lift cables.
Another approach to preventing cable mispositioning has involved utilization of retainers in the form of a hood, shroud or snubber associated with the cable drums. With these devices capturing the cable between the drum and the retainer, the proper cable positioning can be maintained for a particular size drum and system components. However, these retainers do not permit utilization on other than a particular one of the many different drum sizes and configurations employed by different manufactures for different door systems.
Thus, no solution to substantial cable slack in sectional overhead door systems having motor driven counterbalance systems, for cable drums of different designs and sizes, has been recognized in the industry.